UC Davis

Scouring of pile group foundations is a common phenomenon for cross-river bridges and can produce significant damage in earthquake-prone regions. This study experimentally investigated the seismic failure mechanism and postearthquake vertical load-carrying capacity of scoured pile group foundations. Three identical 2 × 3 reinforced concrete (RC) pile group specimens were embedded in homogeneous medium density sand with an overall scour depth equal to five times the diameter of a single pile, and then were subjected to lateral cyclic loads applied to the pile cap in order to produce a predetermined damage state in the piles. Pushover in the vertical-downward direction (pushdown) was finally applied on these damaged specimens exhibiting a permanent lateral displacement to evaluate their residual load-carrying capacities. Experimental results show that the leading pile was more prone to seismic damage, as both the first aboveground and first belowground plastic hinges originally occurred on it. The embedded depth of potential plastic hinges in leading, middle, and trailing piles gradually increased. In addition, the extension of pile damage had a significant influence on the residual vertical load-carrying capacity and the corresponding vertical failure mode of the pile group. Reductions of 10.4%, 47.5%, and 73.8% in the vertical load-carrying capacity of these scoured pile group specimens were recorded when they previously suffered a displacement ductility of 1.75, 3.5, and 5.0, respectively. Based on the experimental results, a linear degradation formula on the normalized postearthquake vertical load-carrying capacity of pile groups with respect to the displacement ductility was developed. The experimental results presented in this paper could be used to validate the ductility capacity and residual vertical load-carrying capacity of pile groups numerically evaluated by using three-dimensional nonlinear finite-element models. This research represents also a first step toward the development of a rapid postearthquake assessment approach for bridges with pile group foundations.